Entering the caves has been described by many as like walking into Superman's cave or a giant geode. However, to do so for any prolonged period you'd probably have to be a superhero. The caves are not only very hot (100-150 degrees Fahrenheit), but they have 100% humidity – making it impossible for a human being to survive inside them for longer than eight minutes.

The smaller of the two is about the size of a two-bedroom apartment, the larger (and hotter) a cathedral. The largest previously known crystals were found in the nearby Cave of the Swords, some of the crystals from which are on display at the Smithsonian Institute.

How did they form? It's believed that the mineral anhydrite, which was abundant in the underground water, dissolved into gypsum and crystalized in the narrow, stable temperature range (136 degrees Fahrenheit). The mine company are still deciding how best to preserve the crystals, so until they know the caves are closed to the public. However geologists believe it is very likely there are further caves like them in the region. RM